1. Field of the Invention
The present invention relates to an apparatus and method controlling a disc drive, and more particularly, to a laser power control apparatus and method in a disc drive, which are capable of preventing fluctuations of laser power due to a variation in the reaction speed of an automatic power controller according to the write speed of a disc drive.
2. Description of the Related Art
In general, optical disc drives, e.g., CD drives, DVD drives, CD-ROM drives, and DVD-ROM drives, write or read data to or from a disc using an optical signal. Such a disc drive writes data to a disc by projecting light output from a laser diode onto the disc. Also, the disc drive reads data from a disc by projecting an optical signal output from the laser diode to the disc, using a lower laser power than that used for writing, converting the optical signal into an electrical signal using a light-receiving element, and processing the electrical signal.
During the writing and reading operations of the disc drive, an automatic power control circuit, which automatically controls a current used to drive the laser diode, is required to maintain a constant power level of an optical signal output to the laser diode.
Referring to FIG. 1, a conventional laser power control apparatus in a disc drive includes a laser diode LD1, a photodiode PD1, a current-to-voltage (I/V) converter 110, an automatic power controller (APC) 120, and a laser diode driving unit (LDD unit) 130.
The APC 120 generates a laser diode driving (LDD) voltage shown in FIG. 2A in response to a writing pulse control signal in a write mode. In detail, in a time interval T1, the APC 120 samples and holds the voltage of a signal output from the I/V converter 110 in response to a gate pulse in the time interval or period T1, compares the voltage with a predetermined read power reference value, and generates an LDD voltage having the same level as the read power reference value. In a time interval T3, the APC 120 samples and holds the voltage of a signal output from the I/V converter 110 in response to a gate pulse in the time interval T3, compares the sampled voltage with a predetermined write power reference value, and generates an LDD voltage having the same level as the write power reference value. In a time interval T2, which is the beginning of the write pulse, the APC 120 causes occurrence of overshoot for a predetermined time.
The LDD unit 130 converts the LDD voltage output from the APC 120 into a driving current and applies the driving current to the laser diode LD1. As a result, the laser diode LD1 outputs laser light.
Next, the laser light output from the laser diode LD1 is reflected from the disc and passes through the photodiode PD1, is converted into a current, which is an electrical signal, and applied to the I/V converter 110. Then, the electrical signal passing through the I/V converter 110 is converted into a voltage signal having a waveform shown in FIG. 2B and applied to the APC 120. Using a control loop of the conventional laser power control apparatus of FIG. 1, the power of laser light output from the laser diode LD1 can be maintained to be constant.
However, when the write speed of a disc drive increases, the reaction speeds of circuit devices of the APC 120 and optical devices such as the laser diode and the photodiode are lower than the reaction speed of the write speed of the disc drive, thereby causing fluctuations in laser power.
As shown in FIGS. 2A and 2B, in a write mode at a low write speed, a voltage of level 1 is output from the I/V converter 110 in a read power interval T1. However, the time period between a read power interval and a write power interval becomes shorter at a high write speed; thus, write power of a write power interval T2 is applied to the I/V converter 110 before the read power interval T1 completely reaches a Level 1. As a result, the I/V converter 110 abnormally outputs a voltage of a Level 2 as read power.
Then, the APC 120 considers the read power, which is applied in a write mode at high write speed, as a voltage of level 2 and generates an LDD voltage to lower the read power level, thereby greatly lowering the level of read laser power.
Accordingly, the conventional laser power control apparatus of FIG. 1 is disadvantageous in that the read laser power level is abnormally lowered in a write mode at high write speed and it is difficult to detect various servo signals, such as a tracking error signal and a focus error signal, thereby deteriorating stable servo control. Also, the level of a detected wobble signal is not constant, and thus, the recording characteristics of a disc drive are degraded.